There are many different factors involved in why edible bananas don't have seeds, there is no simple answer. But here are a few answers.
For commercial Cavendish bananas, they are both female and male sterile, so there is no chance of setting seed.
For backyard growers with lots of different varieties, most edible bananas are only weakly fertile if at all (though some are very fertile), and the chances of seed set decrease as fertility decreases. More over, the pollen doesn't move very easily in most edible bananas, usually because there is a very small amount compared to the wild species and so cross pollination works best by hand.
In general, edible bananas have been inherently selected to be both male and female sterile. Though a wide range of sterility and fertility still exists, the average is that they are more or less sterile. Sometimes a cross pollination event will result in no seeds, other times the same cross may yield a few, there are many factors involved.
Another reason we don't see our female fertile edible bananas full of seeds normally is due to the lack of a pollen source. As stated earlier, edible bananas are not a good pollen source, even though its possible to use some of the pollen from them under controlled conditions. However, if you are growing some wild species around female fertile edible bananas and there is a suitable pollinator around, you can get seeds. I have observed seed set in both 'Orinoco' and 'Ney Poovan' when they were growing around M. balbisiana. In Asia where there can be an abundance of wild bananas, some cultivars which would be otherwise seedless are often very seedy.
Here are some photos of seed set in 'Orinoco' which was growing in close proximity to M. balbisiana. I collected perhaps 30-40 seeds from this fruit bunch.